The Written and Oral Torah are two critical components of Torah. They are inseparable and although the Oral Torah can be reconstructed by the 13 Principles expounded by Rebbi Yishmael (Reish Sefer Toras Kohanim,) Torah by definition constitutes both.
At the same time, the ultimate fulfilment of exertion in Torah is Torah Shebeal Peh (Oral Torah.) The reason why can be discerned by studying the differences and connection between the Pesach and Purim experiences.
“On Purim they fulfilled what they had accepted already” (Shabbos 88b)
What had they accepted?
Torah min hashamayim (from the Heavens)
At best passive human input, at the least coercion.
Yad Hashem - Hand of God
What was the fulfillment of Purim?
Lo bashamayim hi- it is not in the heavens (Devarim 30:12)
He determines the direction to follow thus manipulating the outcome through hidden hashgocha (Divine Providence) – venahapoch hu (it was turned around)(Megilas Esther 9:1.)
Overriding at least one positive mitzvah (eating matzah) and the Jews follow her decree.
The ultimate fulfillment of Torah is Torah shebal peh – the determination of halacha (law) and hadracha (approach) by chachamim (sages) and the emunah (faith/ trust) of people in the chachamim – the human element of Torah.
This is evident at the splitting of the Yam Suf (Read Sea:) veyaaminu b’Hashem uveMoshe avdo (and they believed in Hashem and in Moshe (Moses) His servant,)(Shmos 14:31) virtually equating the trust in Hashem with that in His servants.
Yad Adam - Hand of Man
Turning Yad Adam into Yad H’
Therefore we can see on the deepest level a relationship between Pesach and Purim – the Pur (lot) was drawn in Nissan for the month of Adar. Esther fasted on Pesach and the Jews were redeemed on Purim.
Pesach is Yad Hashem – thus we start the year understanding Hashem is the foundation of everything and end the year putting this knowledge into the world of action. Thus the mizvos (practices) of Purim are all action based.
So we see a beautiful illustration of the symbiosis of the Written and Oral Torah.